Many of today's computers, cellular telephones, and other such devices rely on wireless communication to exchange information with other devices, such as to surf the Internet, send and receive email, and communicate audio and video. Many such devices also include GPS, useful in that it enables a person to determine where they are, overlay their location on a map, and get directions. GPS signals are often also used by other applications, such as a restaurant review application that shows reviews of restaurants near a user, or a shopping application that shows stores near a user that have a particular item for sale.
But, GPS signals are difficult to receive in some cities with tall buildings or other interference, and GPS is often left out of lower cost devices. Some devices therefore use cellular phone towers or other radio signals having known location to use as a reference, and can determine approximate position of a mobile radio device by estimating the distance from the mobile wireless device to multiple cellular towers. This can be performed such as by using chirp signals and measuring the time of flight between RF devices, but accuracy is dependent in part on synchronization between the two devices. Such methods are particularly suited to chirp-modulated communication systems, where the offset of the carrier frequencies between the transmitter and receiver appears as a time offset at the receiver.
In a system where a chirp signal is sent and returned, the outgoing signal can be mixed with the incoming or returned signal to produce a signal with a fixed frequency at the output of the mixer. This frequency indicates the distance between the nodes. In another range finding system, the returned signal is digitized and autocorrelated with the transmitted digital signal to determine the delay between the transmitted signal and received signal.
There exists a need to provide easy and accurate signal delay measurements in wireless systems, such as for radio ranging.